1. Field of the Invention
The present invention relates to spatially resolved differential analytical techniques for determining the composition, phase, structure, or other properties of a sample of material.
2. Background of the Invention
Thermal analysis techniques generally comprise measuring a physical parameter as a function of the temperature of the sample. The sample temperature is strictly controlled throughout the analysis. Whenever the sample undergoes a chemical transformation, a physical transformation, a phase change, or another transition which affects the physical parameter being measured, the changes in that physical parameter may be interpreted to analyze the composition, structure, or thermal stability of the sample.
In differential thermal analysis techniques, the physical parameter of the sample being measured is compared to that of a reference, as a function of the temperature of the sample. The difference in the physical parameter measured for the sample and that measured for the reference is then recorded. The differential thermal analysis technique compensates for the effects of heating rate and ambient conditions that could cause changes in the measured physical parameter of the sample and reference. The differential thermal analysis technique can increase the sensitivity of the measurement of the physical parameter by removing large offsets in the value of the physical parameter whenever the precision of the measuring apparatus is limited.
Proximal-probe techniques such as Scanning Tunneling Microscopy and Atomic Force Microscopy obtain spatially-resolved characterization data by bringing a very small probe very close to the sample surface. These techniques are described, for example, in M. Hietschold, P. K. Hansma and A. L. Wiesenhorn, "Scanning-Probe-Microscopy and Spectroscopy in Materials Science," Microscopy and Analysis, September, 1991, pp. 25-27; and N. F. van Hulst and F. B. Segerink, "Optical Microscopy Beyond the Diffraction Limit," Microscopy and Analysis, January, 1992, pp. 21-23. Companies producing Scanning Tunneling Microscopes include Digital Instruments, Santa Barbara, Calif.; Burleigh Instruments, Fishers, N.Y.; and Struers, Westlake, Ohio.
C. C. Williams and H. K. Wickramasinghe, "Photothermal Imaging with Sub-100-nm Spatial Resolution," published in Photoacoustic and Photothermal Phenomena, Proceedings, P. Hess and J. Pelzl, eds., Springer Ser. Opt. Sci. v. 58, pp. 364-369 (1988) is incorporated by reference herein. This article describes a high resolution thermal microscope using a thermocouple probe.
High resolution analytical techniques are described in U.S. patent application Ser. No. 07/638,847, which is incorporated by reference herein. Those techniques seek to improve the resolution of changes in a characterizing physical parameter by controlling the rate of sample heating during transitions as a function of the rate of change of the physical parameter. When non-differential thermal analysis techniques are used, the high resolution techniques are effective in improving resolution for many transitions. However, they usually reduce the sensitivity of transitions when applied to differential thermal analysis techniques. This is because, for most differential thermal analysis techniques, the magnitude of the differential physical parameter is a direct function of the heating rate. Reducing the heating rate during transitions causes the differential signal to change, which may alter or obscure the true differential signal resulting from the transition event. This obscuring of the physical parameter can reduce the utility of the high resolution techniques when applied to conventional differential thermal analysis techniques.
Conventional differential thermal analysis techniques are limited in their ability to separate non-reversible events caused by enthalpic processes (chemical or physical) from reversible events such as changes in the heat capacity of the sample. This is because the reversible and non-reversible processes often occur simultaneously, or occur severely overlapped in time and/or temperature.
In addition, conventional and high resolution thermal analysis techniques cannot distinguish between rapidly reversible and non-rapidly reversible transitions within a single heating or cooling scan of the sample.